1. Field of the Invention
This invention relates to alarm systems and detectors or peripheral devices such as those that connect smoke alarms to central alarm panels. It has to do typically with fail safe sensitive monitors and signalers for interconnect lines of AC-powered smoke alarms with battery backup.
2. Description of the Prior Art
Alarm systems often contain multiple sensors including smoke detectors, heat detectors, motion detectors, and switches that determine the open or closed state of a door or window. In some systems the individual sensors or a plurality of sensors are monitored by the system using one or more electronic loops, and the system sounds an alarm when one of the sensors is triggered. Such systems and loops are discussed in the prior art (U.S. Pat. Nos. 4,141,007; 4,144,528; 4,162,489; 4,176,346; 4,517,555; 4,191,946; 4,586,028; 4,745,398). In other types of alarm systems, the sensors themselves may contain alarm horns. Such devices may be standalone devices or be interconnected such that if one is triggered, the others also become triggered by an interconnect line. Such circuits are described in the prior art (U.S. Pat. Nos. 4,194,192; 4,207,558; 4,972,181; 4,138,670).
This interconnect mode of operation is available in AC powered smoke detectors with battery back up. One such detector is the model 86RAC manufactured by BRK Electronics of Aurora, Ill. The backup is a 9-volt battery, and when the alarm is activated, something less than 9 volts DC is available on the interconnect line at a maximum current of approximately 6 milliamps. Total available power available may be considerably less if a poorly charged battery is required to power the interconnect line. This smoke detector is sold as a stand alone smoke alarm system with no UL approved means offered by the manufacturer to connect it to a central alarm system panel which could then call an alarm company.
Thus, if someone with AC smoke detectors with battery backup and interconnect capabilities (AC-DC-interconnect detectors) wanted to have smoke detectors connected to a central alarm panel, they would have to buy a different type of smoke detector manufactured for that purpose. These additional detectors are DC powered smoke detectors. There are fundamental differences in the operation of these smoke detectors that are important to appreciate in order to understand the operation of the present invention. Typically, the DC detectors have no horn. When they sense smoke, they communicate this condition to the central alarm panel by producing a short or near short across a loop from the central alarm panel. The loop is created by two leads from the panel, which are joined at their free ends by a so-called "end of line resistor." The central alarm panel circulates current through the loop. The panel expects a predetermined amount of current to be in the loop, based on the loop resistance generated by the end of the line resistor. If a break or high impedance state occurs in the loop, the panel recognizes the decrease or loss of current and reports this "trouble state" to the alarm user as a unique signal that differs from the signal that would be used if the detector sensed smoke. The signal may be audible or visual or both. If the central alarm system panel is so equipped, it will also call an alarm company and report the condition.
Leads from the DC smoke detector(s) make contact with the loop such that the detector is wired in parallel with the end of line resistor. Thus, when smoke is detected and the detector shorts the loop, the central alarm panel notes the change in the loop current from the predetermined value to a much higher current which is limited by resistance in the circuitry supplying current to the loop. The central alarm panel then activates a unique audible and/or visual signal for the alarm user and, if equipped, calls an alarm company.
Thus, when connecting DC smoke detectors to a central alarm panel, one uses a loop from the panel, and the loop is of special type that can appreciate three different states. In the "no alarm, no trouble state" a predetermined current is noted circulating through the loop. In a "trouble state," low current is noted in the loop. In the "alarm state" high current is noted in the loop. This type of loop will be referred to in the specification as a "smoke detector loop." This designation will distinguish it from the other type of loop typically offered by central alarm panels--the "normally closed loop." This latter type of loop is supplied with a predetermined current from the central alarm panel that senses the presence or absence of the current. Such loops are typically wired with a switch in series. An example is a magnetic switch on a window that maintains a closed loop when the window is closed and an open switch when the window is open. Such loops cannot distinguish a "trouble state" from an "alarm state" as both conditions produce the same result, namely, opening the loop.
The AC-DC-interconnect detectors are inexpensive and sometimes required by building codes in new constructions. The DC smoke detectors are considerably more expensive and if added to the system will add substantial cost. It would thus be advantageous to have a circuit available that would allow one to interface the AC alarms to a central alarm system panel.
This might be accomplished by a relay that would open a normally-closed loop from a central alarm system panel when the interconnect line goes hot. There are, however, two problems with that approach.
1. Conventional relays would not be able to be powered by minimal battery power should the AC power fail. PA1 2. The relay would never be opened if the interconnect line was disrupted between the active alarm and the relay.
A desirable circuit would thus be capable of being powered by a weak battery and be activated by discontinuity of the interconnect line. Furthermore, the circuit should be fail-safe, which means that failure of any component in the circuit would result in opening of the alarm loop. This is a quality that would be advantageous if not necessary in obtaining a listing from the Underwriter's Laboratory (UL). Such listing would almost certainly be required in order for contractors to install the device.
Line monitors and fail safe circuits are known, but none are designed as stand alone circuits with the inherent simplicity of the present invention or for the same type of purpose--the interfacing of AC smoke detectors having battery backup and interconnect capabilities to a central alarm system panel. That interconnect lines were described for use in self powered smoke alarms (U.S. Pat. Nos. 4,194,192; 4,207,558) more than 15 years ago and that to date no UL-APPROVED device to couple those interconnect lines to central alarm panels has been made commercially available speaks for the lack of obviousness of the circuitry of this invention.